1. Field of the Invention
The present invention relates to a hermetic sealing method and, more particularly, to a hermetic sealing method, which is capable of preventing oxidation of a micro-electromechanical system (MEMS) and sealing the MEMS at a low temperature.
This application is based on Korean Patent Application No. 2001-76235 filed on Dec. 4, 2001, the disclosure of which is incorporated herein by reference in its entirety.
2. Description of the Related Art
In general, a micro-electromechanical system (MEMS) is manufactured through photolithography, deposition, and etching after a very thin metal layer or a thin film, such as oxide and polymer, is laminated on a silicon substrate. The MEMS is very thin and microscopic, and thus is very sensitive to the environments of high temperature, fine dust, water or gas. In particular, a capillary phenomenon due to water or by-products, and a surface force, such as Van der Waals force, causes adhesion. Thus, in order to prevent adhesion, it is necessary to maintain a predetermined atmosphere in a cavity of a package. In order to maintain this atmosphere, the package must have definite sealing properties, and thus, various hermetic sealing methods have been developed.
Methods for sealing the MEMS include a wafer level bonding method and a chip level sealing method. However, a device such as a micromirror that is employed in the MEMS has weak heat resistance properties and is very susceptible to an environment of adhesion due to water or by-products, and thus, taking into consideration the characteristics of the MEMS, the most appropriate sealing method must be selected.
The wafer level bonding method includes an anodic bonding method, a glass and glass frit bonding method, an adhesive bonding method, and a low melting eutectic bonding method. In the anodic bonding method and the glass frit bonding method, the sealing is performed at a high temperature of over 400° C. As described previously, the device such as a micromirror that is employed in the MEMS has very weak heat resistance properties, and thus, the anodic bonding method and the glass frit bonding method that are performed at a high temperature are not suitable for sealing the device. In the adhesive bonding method, due to the penetration of water contained in an adhesive as well as water penetrated from an external environment, perfect hermetic sealing is not possible.
In addition, there is the chip level sealing method, which includes a seam welding method, a soldering method, and an adhesive bonding method. In the seam welding method, damages to a device due to heat can be comparatively reduced, and hermetic sealing is possible, but the components and the facilities required for the seam welding method are costly. On the other hand, in the sealing method by Sn—Pb soldering, hermetic sealing is possible at a comparatively low price, but Sn—Pb is melted at a comparatively high temperature (about over 183° C.) in a MEMS manufacturing process, and wetting defects due to an oxide layer that is formed in the MEMS manufacturing process become sealing defects, and an environmental problem due to a Pb component occurs.
More specifically, FIG. 1 illustrates a conventional semiconductor device package disclosed in U.S. Pat. No. 5,550,398. An optical window 101 having a metal layer 102 is attached to a metal frame 104 by a solder preform 103 and is sealed in a housing 105, such as a ceramic package. The solder preform 103 is made of Au—Sn, which is a costly process that also requires a proper cleaning so as to prevent contamination and oxidation.
The optical window 101 is manufactured by soldering in the atmosphere of gas injected during sealing, a Kovar ring is brazed in the housing 105, and the optical window 101 is sealed in the housing 105 by a seam welding or a laser welding. The process is very complicated and costly to manufacture.
FIG. 2 illustrates a conventional method disclosed in U.S. Pat. No. 6,046,074. First through third thin film sealbands 112, 113 and 114 used for wetting a solder are sequentially deposited on a substrate 111 on which a chip c is formed, and on a cap 116. Then, solder 115 is disposed between the cap 116 and the substrate 111, is heated and sealed. The solder 115 is provided with preform having a band shape, and is formed of Sn—Pb or Pb-free solder, which is an alloy made up of In, Ag, Sb, and Bi. However, a Pb or Cd-family solder contaminates an environment, and thus, Pb-free solder has been widely used. Among the Pb-free solders, In has a comparatively low melting point of about 156° C., and alloy of Sn, Bi, Ag, and Zn plus In has a comparatively low melting point of about 118–170° C. and a high ductility, and thus is a suitable material for hermetic sealing of a device with weak heat resistance properties, such as a MEMS.
However, the Pb-free solder is easily oxidized after deposition or during sealing, and thus, in order to prevent this, the Pb-free solder must be etched with a strong acid such as HCL, or dry-etched. Thus, the manufacturing process becomes complicated. The difficulty in manufacturing is that the Pb-free solder must be immediately sealed so as to prevent the Pb-free solder from being re-oxidized after etching. Also, defects may occur in a device after sealing due to water or a by-product, which is generated during etching. Further, when the Pb-free solder is heated at a high temperature during sealing, an oxide layer is formed, and thereby, defects in wetting, voids and cracks occur and a perfect hermetic sealing cannot be performed.
A flux is used to remove the oxide layer, but the flux is formed of acid rosin, and thus oxidizes and damages Al and other corrosive metals of a MEMS, and thus, adhesion occurs due to a by-product generated by the flux.
FIG. 3 illustrates a conventional method disclosed in U.S. Pat. No. 6,062,461. A solderable ring 118 is formed around a wafer 119 on which a MEMS device 121 is formed, and a solderable layer 117 is formed on a part of the upper surface of a capping wafer 120 or on the entire upper surface of the capping wafer 120. Then, the solderable ring 118 is joined to the solderable layer 117 and is soldered at a low temperature. Here, the solderable layer 117 is formed through plating, evaporation, and sputtering and is comprised of Sn—Pb or Pb-free solder, which is an alloy made up of In, Sn, Ag, Sb, and Bi. However, since this method also uses the Pb-free solder, problems due to oxidation described earlier remain.